Preheat the oven to 375°F. Boil a pot of salted water and cook the spaghetti until al dente; drain. While the pasta cooks, melt the butter in a large skillet over medium heat and sauté the onion for 5 minutes until soft, then stir in the garlic for 30 seconds.

Reduce the heat to low. Stir in the cream of mushroom soup, milk, and Velveeta cubes. Cook, stirring often, until melted and smooth, about 5 minutes. Add the undrained Rotel, paprika, and a pinch of salt and pepper.

Remove from the heat. Add the drained spaghetti and shredded chicken, folding gently until everything is evenly coated.

Transfer to a greased 9x13-inch baking dish and spread into an even layer. Bake for 20 to 25 minutes until bubbly and lightly golden on top. Let cool 5 minutes, then serve.